Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as methanol-to-olefin(s) (MTO) process, where methanol is converted to ethylene and propylene in the presence of a molecular sieve.
The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. An important type of alternate feed for the production of light olefins are oxygenates, such as alcohols, particularly methanol and ethanol, ethers such as dimethyl ether, methyl ethyl ether, and diethyl ether, dimethyl carbonate, and methyl formate. These oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or other organic materials. One process that is particularly useful in producing olefins is the conversion of methanol to hydrocarbons and especially to light olefins. The commercial interest in the MTO process is based on the fact that methanol can be obtained from readily available raw materials such as coal or natural gas which are treated to produce synthesis gas which is in turn processed to produce methanol.
Oxygenates are converted to an olefin product through a catalytic process. The conversion of a feed containing oxygenates is usually conducted in the presence of a molecular sieve catalyst. Although ZSM-type molecular sieves and other molecular sieves may be used for the production of olefins from oxygenates, silicoaluminophosphate (SAPO) molecular sieves have been found to be of particular value in this catalytic process.
Silicoaluminophosphate molecular sieves are manufactured from sources of silicon, such as a silica sol, aluminum, such as hydrated aluminum oxide and phosphorus, such as orthophosphoric acid. In addition, an organic template such as tetraethylammonium hydroxide, isopropylamine or di-n-propylamine is used. SAPO-34 belongs to the family of molecular sieves having the structure type of the zeolitic mineral chabazite (CHA). The CHA framework type has a double six-ring structure in an ABC stacking arrangement when viewed perpendicular to the rhombohedral 3-fold axis.
The preparation and characterization of SAPO-34 has been reported in several patents including U.S. Pat. Nos. 4,440,871 and 5,248,647, both of which are herein fully incorporated by reference.
One of the most important embodiments of the MTO conversion process is directed to the production of light olefins, i.e., olefins containing from 2 to 4 carbon atoms, inclusive. Accordingly, it is important to utilize a catalyst which maximizes the production of these products, results in a high degree of conversion of the starting methanol, and does not deactivate rapidly under the process conditions imposed. In the conversion of methanol to olefins, SAPO-34 exhibits relatively high product selectivity to ethylene and propylene, and low product selectivity to paraffin and olefin with four or more carbons (C4+olefin).
The effect of the particle size of the molecular sieve on activity has also been documented in U.S. Pat. No. 5,126,308. In the '308 patent, it is disclosed that molecular sieves in which 50% of the molecular sieve particles have a particle size less than 1.0 μm and no more than 10% of the particles have a particle size greater than 2.0 μm have increased activity and/or durability. The '308 patent also discloses that restricting the silicon content to about 0.005 to about 0.05 mole fraction also improves catalytic performance.
One desirable group of silicoaluminophosphate molecular sieves is those that have low silicon content. Silicoaluminophosphates of the CHA framework type with low silicon content are particularly desirable for use in the MTO process. Low silicon content has the effect of reducing propane formation and decreasing catalyst deactivation. However, it has proven difficult to make pure phase CHA silicoaluminophosphate molecular sieves with low silica to alumina ratio.
In the art, various attempts have been made to improve the synthesis of AlPO4 or SAPO molecular sieves. One approach has been the addition of a source of fluoride ions to the synthesis mixture. However, this approach has the disadvantage that many of the fluorides cause cost, safety or environmental concerns due to their toxicity, corrosiveness and volatility. It would be highly desirable to have a process that avoids their use. U.S. Pat. No. 6,620,983 B1 describes the use of other fluorine containing compounds. These compounds have two or more fluorine substituents, as the source of fluoride ion, in the synthesis of aluminophosphates or silicoaluminophosphates. Although the molecular sieves produced are described as having the desired chabazite crystal structure, they produce a lower than desired yield of light olefins when used in a methanol to olefins process. Therefore, these other fluorine containing compounds are not the solution sought. It would also be desirable to have a more effective catalyst in conversion of oxygenates to olefins.
It is therefore desirable to find new processes, which are specific for the synthesis of molecular sieves having the CHA framework type. A particular need is to find methods of preparing low silica SAPO molecular sieves, which do not require the use of hydrogen fluoride or other fluorides.